ORGANS OF RESPIRATION. 559 



as they are in quadrupeds, there can yet be no muscular septum 

 or diaphragm separating its cavity into a thoracic and an 

 abdominal portion. 



As in most reptiles, the pulmonary organs of birds are 

 composed of an anterior minutely subdivided cellular portion, 

 and a posterior largely sacculated part, but slightly partitioned 

 by internal septa, and contained, as usual in oviparous verte- 

 brata, in the same cavity with the digestive organs. The 

 highly vascular parietes of the abdominal air-sacs, or enlarged 

 pulmonary cells, covered with peritoneum, and lined with 

 the ciliated mucous membrane of the lungs, extend forwards 

 and backwards around the chylopoietic viscera, and reach in 

 every direction beyond the cavity of the trunk, as into the 

 axilla, the neck, the vertebrae, and the bones of the extre- 

 mities. Their interior is partially subdivided by imperfect 

 septa, the first rudiments of all pulmonary cells, which begin 

 to develop from the periphery ; and they communicate by a 

 few large round openings, on the inferior surface of the lungs, 

 with the ends of the wide bronchial tubes, which traverse these 

 organs, with little subdivisions, and nearly in a straight 

 course. 



These air-sacs, and their free communications with the lungs 

 and trachea, were familiar to Harvey, who regarded them 

 as the principal parts of the lungs of birds, and who found 

 their bronchial openings in the ostrich so wide as to admit 

 the finger. The air-sacs are generally in contiguous pairs, 

 separated by a median septum, and opening each by a dis- 

 tinct foramen into its corresponding lung. One large 

 cell on each side extends forwards, under the sternum and 

 coracoid bone, supplying with air the bones of the shoulder 

 and arm, enveloping the heart, the bronchi, and the inferior 

 larynx, and extending into the axilla. Another extends into 

 the neck above the clavicles, forming various enlargements in 

 different species, on each side of, or beneath the crop ; ano- 

 ther beneath the sternum, extends also behind the heart and 

 oesophagus, to the vertebrae of the neck, thus enveloping the 

 great vascular trunks of the heart. Another large pair, some- 

 times subdivided into an anterior and posterior compartment, 

 extends backwards along the dorsal and lateral parts of the 

 abdomen, enveloping most of the viscera, forming air- sacs 

 in the pelvis, and extending into the bones of the legs. 



The bones of the head, neck, trunk, and extremities, are thus 



